This invention relates generally to microwave ovens and more particularly to microwave ovens adapted to operate with alternate line voltages.
As it is known in the art, power distribution varies in the United States and in foreign countries and often varies even within a country. For example, power distribution in the United States servicing commercial business locations generally provides a nominal voltage of either 208 VAC or 240 VAC.
As it is also known in the art, microwave ovens include a magnetron which generates energy at microwave frequencies suitable for cooking. Microwave ovens further include a power supply, having an output coupled to the magnetron, such power supply providing high voltage, on the order of several thousand volts, to the magnetron. The power supply generally includes a step-up or power transformer, and it is desirable that, regardless of whether the microwave oven is supplied with a nominal line voltage of 208 VAC or 240 VAC, the same voltage be provided across the secondary winding. The power supply should also maintain the same voltage across the secondary winding when the line voltage varies over standard ranges or tolerances, such tolerances typically being on the order of +10/-15% and caused by load variations. Providing the same voltage across the secondary winding regardless of the voltage across the primary winding is necessary in order to regulate the output power of the magnetron to within a predetermined range since, if the output power were higher or lower than expected, the preset cooking times for the various cooking programs would result in the food being undercooked or overcooked.
One technique known in the art for providing the same high voltage at the output of the power supply, regardless of whether the microwave oven is connected to a 208 VAC or 240 VAC line voltage, is to provide a series of jumpers or switches which, when set appropriately, configure the power supply, and specifically the power transformer, for either 208 VAC or 240 VAC operation. More specifically, the step-up transformer of the power supply is provided with a plurality of different input connections on the primary winding, and, depending on the line voltage, the line is connected to the appropriate input connection. However, the use of such jumpers or switches to modify the microwave oven for operation with different line voltages generally requires a service technician to set the jumpers or switches appropriately. Further, the service technician generally has to visit the site of the microwave oven installation so that the line voltage, if not known, can be measured to determine the proper settings for the jumpers or switches. However, such a required service visit generally increases the overall cost of the oven as well as the time of installation for the oven.